Seismic response characteristics and reservoir distribution prediction of karst reservoirs in the Yingshan Formation of the Lunguxi block

GUAN Baozhu; LIU Junfeng; YANG Maochuan; YANG Xinying

doi:10.11932/karst20250214

Abstract：Globally,279 carbonate oil and gas reservoirs have been discovered across 39 countries and regions.Notably,large marine carbonate oil and gas reservoirs are primarily distributed in China,mainly featuring lithological-stratigraphic traps.The buried depth of oil and gas reservoirs is generally greater than 5,500 m,and they are mainly concentrated in large oil-and-gas-bearing basins,such as the Tarim Basin,Sichuan Basin,Ordos Basin,etc.The rapid development of China's petroleum industry has resulted in a growing volume of crude oil imports each year.The increasingly complex international landscape has posed challenges to the stability of China's petroleum supply.The exploration and development of carbonate karst oil and gas reservoirs is one of the ways to ensure a stable energy supply.The relevant areas have been transforming from strategic oil reserve zones to regions for oil resource replacement.Therefore,accurate prediction and precise evaluation of carbonate karst reservoirs are increasingly receiving attention from petroleum geologists.Lunnan ancient buried hill is located in the central area of the Lunnan low uplift within the Tabei uplift of the Tarim Basin,which is the main part of the Lunnan paleo-uplift in the Tarim Basin.The Lunguxi block is located in the west of the Lungu oil and gas field.In the Lunguxi block,especially in its northern area,many wells have been drilled;however,their exploitation has been inefficient and unsuccessful.The low drilling rate of the target layer may be the main reason for these phenomena.Therefore,it is urgent to address the challenges of accurate prediction and precise evaluation of carbonate karst reservoirs. According to the characteristics of karst fracture-cavity development and the karst hydrodynamic conditions,and in conjunction with modern karst geological theory,carbonate karst reservoirs can be classified into three main types,surface karst reservoir,underground river karst reservoir,and fault-controlled karst reservoir.High-angle structural fractures,dissolution pores,and small caves are mainly developed in surface karst reservoirs.The dissolution pores and fractures are primarily connected vertically,with the potential for lateral interconnection over short distances along the fractures.The fractures and small-scale caves are distributed in a planar arrangement and exhibit a strong capacity for adjustment.Underground river systems of karst reservoirs,including underground rivers,sinkholes,vertical shafts,skylights,hall-type and corridor-type caves,underground river outlets and inlets,etc.,exhibit good connectivity along the direction of groundwater runoff.Existing wells for exploitation show that the underground river karst reservoirs are the main reservoir type for oil and gas production,and are also one of the most important reservoir types in the study area.The development of fault-controlled karst reservoirs is influenced by fault activities.These reservoirs are formed by the dissolution and transformation of diagenetic fluids along fault surfaces or structural fractures,resulting in a collection of fracture-cavity complexes.They are developed vertically along fault zones,with a large range of depths,good vertical connectivity,strong lateral heterogeneity,mainly characterized by dissolution fracture-cavities. The carbonate reservoirs in the Ordovician Yinshan Formation of the Lunguxi block exhibit a complex and diverse pore system,with multiple coexisting and interacting geological factors,resulting in strong reservoir heterogeneity.The seismic reflection anomaly of the small fracture-cavity is weak or not easily discernible,exhibiting high concealment,which complicates identification.The development characteristics of low-order faults within faults and ancient river channels,as well as their relationship with the small fracture-cavities,are challenging to quantify.Therefore,based on previous exploration and development data,this study aims to clarify the reservoir types in the study area and subsequently elucidate the seismic response characteristics of these reservoirs.The results show,(1)The seismic characteristics of surface karst primarily exhibit chaotic strong reflections,typically found in the high part of the residual hills and distributed within 0 to 40 meters of the weathering surface of the ancient buried hills.In contrast,the seismic characteristics of underground river karst are characterized by continuous strong reflections in the seismic profile,with the impedance attribute presenting as a low-value anomaly.This anomaly has a broad lateral distribution range and displays typical river channel characteristics.Additionally,the fault-controlled seismic characteristics are predominantly represented by beaded and chaotic reflections that are distributed along the fault.These reflections exhibit large longitudinal extension and linear distribution patterns along the fault line in a plane view.(2)The seismic response prediction of karst reservoirs in the study area has been conducted from various scales and directions.The research findings indicate that the combination of amplitude attributes and AFE attributes can well characterize the surface karst reservoirs developed in the high part of the buried hill.Additionally,attributes such as differential amplitude and root mean square can systematically identify underground river karst reservoirs.Furthermore,spectrum decomposition discontinuity detection can elucidate the internal details of the fault fracture zone,providing an optimal approach for predicting fault-controlled karst reservoirs based on seismic response.(3)The surface karst is generally developed in the Lunguxi block,particularly in the structurally elevated parts.The fault-controlled karst reservoir is mainly developed in the area of Lungu Wells 9-40.In the northwest and central regions,the thickness of surface karst in karst valleys and slopes is relatively small,whereas in the southeast region,the thickness of surface karst on the karst platforms is substantial.The identification of fault-controlled karst reservoirs can be accomplished through the integration of amplitude attributes and structural tensor attributes.